RESIDENTIAL BY DAVID SHAPIRO
Here’s a problem: Category PJAZ on
page 370 of the 2015 UL White Book,
( www.ul.com/whitebook). tells us that
the armor of the usual, interlocked-armor MC, formally Type MC-I, is not a
grounding conductor. “Metal = grounding” is a myth. When I tell people that
the armor of standard MC isn’t listed
as a grounding means, they agree, but
add that “the armor is metal.” MC-I is
good enough for them. They are not
interested in paying for smooth-armor
or corrugated-armor MC, which is listed
as a ground. Similarly, they are not interested in paying a whole lot more than
they would for standard BX to buy BX
with a separate grounding conductor,
used for isolated-ground circuits.
There is another option: Type MC-IA,
the relatively new MC variety with a bare
conductor snug against the armor. However, the bare conductor is not brought
to a terminal. The combination—not the
armor by itself or the bare aluminum
wire by itself—serves as the grounding
conductor in type MC-IA.
Electricians who chose MC-I over
AC or BX often did so, at least in part,
because they like having a wire-type
bonding conductor that is terminated at
each end at a dedicated bonding terminal.
Would they rely on a connection such as a
locknut or spring clip that provides both
support and bonding? No. They figure
grounding continuity is too important to
take chances on something that has extra
opportunitiess to wiggle loose.
Why, then, did some contractors
migrate to using MC-IA rather than BX,
which has been around so much longer?
There might have been price advantages.
They might have needed MC’s ability to
contain more conductors than BX or different ones. They might have preferred,
say, the greater distances between support
points permitted by Article 330, metal-clad
cable, than by Article 320, armored cable.
Then there are more instinctive motivations: They might have simply gotten
used to MC, seen a promotion for MC-IA,
found the idea of saving labor at each end
attractive, or not even considered BX.
They might have trusted MC-IA’s armor,
with a substantial bare conductor pressing against the inside, better than they did
the armor of BX with its bare 18-gauge
aluminum tape. MC-IA’s bare wire sure
looks as though it could carry fault current from one end to the other, while the
skimpy tape clearly cannot.
But MC-IA users are in the minority.
Most MC users clearly like MC-I better
than BX or MC-IA, often for the reason I
mentioned above: because they are taken
with the idea of having a separate ground
wire plus metal armor.
This brings us back to the myth. MC-I’s armor is not a grounding conductor,
even though the Code requires us to bond
MC at the ends, and bonding can look a
whole lot like grounding. UL’s long-term
representative John Cangemi reminded
me that National Electrical Code (NEC)
Section 300.10, electrical continuity of
metal raceways and enclosures, applies
to cable armor.
Electrical continuity means we have
to bond, though there are two rare exceptions, the first for some short sections and
the second for feeding some isolated enclosures. The requirement has been around
just about forever. With a quick check,
I found it in the 1940 NEC, as Sections
Penetrating an MC myth
3053 and 3071; I also found its ancestor in
Section 505c of the 1925 NEC. Everybody
reading this should know that metal race-
ways and armor need to be continuous.
The Section 300.10 requirement for
continuity, grounding or no grounding,
led me to wonder: If a metal fastener
penetrates both the armor and the
energized conductor it was intended to
protect, how likely is it to trip the over-
Next month, I’ll discuss what else this
could mean, what the MC standard leaves
out and share stories from the field.
Knowing What to Trust
MANY ELECTRICIANS trust standard interlocked-armor metal-clad (MC) cable more
than they do nonmetallic (NM) simply because of the metal armor. It’s harder to
put a nail through armor than through a 30-mil plastic sheath. Many have more
confidence in standard MC than standard modern armored cable (informally BX)
because the MC they use has a separate ground wire. Bringing wire to a grounding
terminal seems like a more secure means to ensure continuity than relying on a BX
connector that has to both hold the cable to the enclosure and bring the ground in
by the bonding enclosure and cable. These same installers may also run a bare or
green-insulated wire inside EM T (thinwall conduit) and do this outside of locations
where it is a Code requirement.
SHAP IRO, author of “Old Electrical Wiring: Evaluating, Repairing, and Upgrading Dated
Systems,” is a contractor, consultant, inspector and writer/editor based in Colmar Manor, Md.
He also is affiliated with IAEI. He can be reached at email@example.com. SH